The units might actually have been
mGy•cm^2 (rather than
mGy/cm^2). This is the dose (in Grays) multiplied by the area it is administered over and is known as the Dose Area Product. This is used as it correlates well with the actual dose delivered to the patient and can easily be measured (or closely approximated) using an ionisation chamber in the path of the beam. For example, a 5x5cm x-ray field with an entrance dose of 1 mGy will have a DAP of 25 mGy•cm^2.
The Gray (Gy) is a (relatively) new international system (SI) unit of radiation dose, expressed as absorbed energy per unit mass of tissue.
The SI unit "gray" has replaced the older "rad" designation. 1 Gy = 1 Joule/kilogram = 100 rad.
The gray is useful clinically and is the main unit used when planning radiotherapy in radiation oncology (a total dose is given in Grays, and administered in multiple fractions to mimimise toxicity.
Although the gray can be used for any type of ionising radiation, it does not account for biological effects. This is where sieverts are used. The dose in gray is multiplied by a factor depending on the radiation.
I found this converter to convert DAP in Gy•cm^2 into sieverts, based on the type of x-ray examination, though it does not have dental x-rays!